1. Field of the Invention
The present invention generally relates to telecommunication devices, and more particularly to an apparatus and method for providing improved DSL communication in an environment wherein both POTS and DSL communications occur over a shared two wire pair.
2. Discussion of the Related Art
In recent years telephone communication systems have expanded from traditional plain old telephone system (POTS) communications to include high-speed data communications as well. As is known, POTS communications includes not only the transmission of voice information, but also PSTN (public switched telephone network) modem information, control signals, and other information that is transmitted in the POTS bandwidth, which extends from approximately 300 hertz to approximately 3.4 kilohertz.
Prompted largely by the growth in Internet usage, the provision of xDSL services to customer premises has proliferated over recent years. In this regard, the descriptor "x" preceding the DSL designator is used to broadly denote a variety of DSL services, including ADSL, RADSL, HDSL, etc. As is known, xDSL transmissions are sent to customer premises over the same twisted pair cabling as POTS transmission are sent. Since xDSL transmissions are communicated in a frequency band that is separate and distinct from the POTS frequency band, transmitting both types of signals over the same cabling (even at the same time), generally is not a problem. Specifically, the POTS frequency band is defined between approximately DC and approximately 4 kHz, while xDSL frequency bands (although they vary depending upon the specific service) are generally defined by a lower cutoff frequency of approximately 26 kHz, and an upper cutoff frequency that depends upon the particular xDSL service.
However, it is known that some measure of additional protection is generally necessary, when the communications occur simultaneously. In this regard, a person speaking into a telephone handset will generally observe audible noise over the handset, even though the xDSL signals are at frequencies above the audible range. It has been generally determined that this audible noise is a result of circuit components within a telephone handset reacting with the xDSL signals to generate lower frequency noise signals, also called intermodulation products. Therefore, some level of additional protection must be provided in order to satisfactorily transmit both POTS information and xDSL signals at the same time.
This additional protection is usually provided by placing a POTS filter at the customer premises. As the name suggests, a POTS filter is a low pass filter that rejects signals at frequencies higher than the POTS frequency band (thus filtering the POTS band signals). Likewise, most xDSL equipment includes a front-end high-pass filter to reject lower frequency POTS signals (as POTS signals tend to distort the xDSL signals by clipping at the analog to digital converter). In this way, xDSL signals are isolated from the POTS signals, which can then be individually routed to the appropriate locations at the customer premises. There are, however, at least two specific manners in which the POTS filter is provided at the customer premises.
First, the local loop is often tapped at the point of entry into the customer premises, and the POTS filter placed at the point of entry. The output of the POTS filter can then be electrically connected to the wiring entering the premises so that all jacks within the premises receive the protection of the POTS filter. Unfortunately, since dual lines are generally not prewired at a customer premises, the second line carrying the xDSL signals must be routed into the customer premises (which may require drilling through walls or other measures). Sometimes this does not pose a significant problem, particularly when the xDSL line need only be routed to a single location that is relative easy to route the line. However, if access to the xDSL service is desired at multiple locations within the customer premises, then the routing of the signal lines will become more tedious, time consuming, and expensive.
A second, relatively simple solution to implement is to provide a POTS filter at every location within the customer premises supporting POTS equipment (e.g., telephones, facsimile machines, PSTN modems, etc.). This filter may be provided in a single housing that plugs into the jacks (e.g. RJ-11 jack). Thus, such a filter will need to be provided for every telephone used in the premises. While simple to implement, this solution certainly imposes a significant cost burden on the customer.
Even though it is a relatively simple task to install individual filters at all POTS devices within a customer premises, it has been found that this is often not done. Whether the reason is cost, difficulty in obtaining filters, or some other reason, often customer premises are equipped with xDSL devices that share a common two wire pair with POTS devices, without any protection beyond the natural protection provided by the dead band that separates POTS from xDSL.
Some users may not install individual POTS filters because they do not hear noise (i.e., intermodulation products) from xDSL devices. That is, if the interference from the xDSL device(s) is not manifest as "audible" noise to a user on a POTS device (e.g., telephone), then the user often does not believe that any extra protection is necessary. However, frequently the dead band separation is not sufficient, even though audible noise is not manifest on POTS devices. For example, lower frequency POTS signals may interfere with xDSL band, thereby corrupting xDSL transmissions. When data is being transmitted, and it gets corrupted, error correction techniques will correct the problem. Typically, however, this correction is in the form of identifying corrupted blocks, and re-transmitting those blocks. Unfortunately, this results in a lower overall data transmission rate.
Accordingly, an improved apparatus and method are desired that overcome the above shortcomings of the prior art. Namely, an improved xDSL communications device that achieves a higher overall data rate when POTS communications are taking place simultaneously over a shared two pair.